On Buzz Ivanhoe's copy of NBS Special Publication 631, p.140-141. M.K. Hubbert, 1982, there is an undated handwritten note:

To L. F. Ivanhoe, a kindred soul, M. King Hubbert

DR. HUBBERT (in response to remarks by David Nissen - Exxon): Your kind remarks with regard to my previous studies of the evolution of the U.S. petroleum industry are greatly appreciated. However, you suggest that my estimates of the ultimate amounts of oil to be recovered is questionable for reasons of classification and because I have not taken into account the effect of the price of oil on ultimate recovery. You mention oil shale, coal, and the Orinoco heavy oils of Venezoela.

With regard to classification, if unintelligibility is to be avoided it is essential that one define his terms and then adhere rigorously to those definitions. In the present study I have been concerned with the techniques of estimation as applied to conventional crude oil and natural gas in the U.S. Lower-48 states. This excludes consideration of shale oil, coal, Orinoco heavy oils, natural gas from unconventional sources, and also oil and gas from Alaska.

My analyses are based upon the simple fundamental geologic fact that initially there was only a fixed and finite amount of oil in the ground, and that, as exploitation proceeds, the amount of oil remaining diminishes monotonically. We do not know how much oil was present originally or what fraction of this will ultimately be recovered. These are among the quantities that we are trying to estimate.

Your statement that the fraction of the original oil-in-place that will be recovered is correct, but the effect may easily be exaggerated. For example, we know how to get oil out of a reservoir sand, but at what cost? If oil had the price of pharmaceuticals and could be sold in unlimited quantity, we probably would get it all out except the smell. However there is a different and more fundamental cost that is independent of the monetary price. That is the energy cost of exploration and production. So long as oil is used as a source of energy, when the energy cost of recovering a barrel of oil becomes greater than the energy content of the oil, production will cease no matter what the monetary price may be. During the last decade we have very large increases in the monetary price of oil. This has stimulated an accelerated program of exploratory drilling and a slightly increased rate of discovery, but the discoveries per foot of exploratory drilling have continuously declined from an initial rate of about 200 barrels per foot to a present rate of only 8 barrels per foot.